Halogenated polymers such as polyvinyl chloride (“PVC”) are employed as building materials to replace wood in a variety of applications such as house fascia, trim, and decorative molding mill work. “Halogenated polymers” as used herein means (1) homopolymers or copolymers containing greater than 80% of vinyl chloride, vinyl fluoride, vinylidene chloride, vinylidene fluoride, or combinations and (2) chlorinated polyvinyl chloride, chlorinated polyethylene, or combinations thereof. The most common of these polymers industrially is polyvinyl chloride (PVC) so the general description herein will emphasize PVC and foamed PVC as examples. PVC foam is also used for signage, deck boards, and in the cores of some types of PVC pipe.
Foamed PVC for these various applications is typically made in a continuous extrusion process. The most common extrusion practices involve free foaming out of the die followed by some type of calibration and the Celuka or integrated skin process. A description of these PVC foaming processes and typical formulation ingredients can be found in D. Klempner. and V. Sendijarevic, “PVC Foams”, Chapter 9, Handbook of Polymeric Foams and Foam Technology, 2nd Ed., Hanser Publishers, Munich, (2004).
Key components of foamed PVC formulations are PVC, thermal stabilizer, lubricants, one or more blowing agents, and (co)polymers additives such as impact modifiers and processing aid polymers. The processing aid polymers are materials that are compatible with PVC and tend to be copolymers that are high in methyl methacrylate or other compositions that are compatible with PVC, for example, styrene acrylonitrile copolymers. U.S. Pat. Nos. 2,646,417, 3,975,315, 5,206,296, and 6,765,033 and EP1153936 describe the types of polymer compositions used as processing aids for PVC. “Compatible” as used herein means that the processing aid polymer mixes or disperses uniformly into the PVC during thermal processing.
High molecular weight processing aids provide polymer expansion or die swell during polymer processing when the heated polymer exits the extruder die. This expansion is important in processes such as the Celuka process in which polymer expansion is required to fill the cavity or in free foam where a certain sheet thickness is required. These processing aid polymers also increase melt extensibility and strength due to their high molecular weight and compatibility with PVC. This in turn helps control the foam cell expansion and provides a small uniform cell size. Additionally, high melt strength helps prevent foam collapse while the extruded foam sheet is cooling and helps lock in the foam structure. High melt strength, in addition, allows the pulling of hot extruded material through sizing or calibrating equipment. Any scrap or trim material can be ground up and reused in the extrusion process in that the foamed material is a thermoplastic and not a cross linked thermoset material. Being able to recycle the material as regrind is important for economics and waste handling.
It is not unusual for these processing aids to have weight average molecular weights in the 0.5 to 15 million range with the higher MW materials showing greater efficiency (Haworth, B., Chua L., Thomas, N L., Plastics, Rubber and Composites Processing and Applications, (1994), V. 22, p. 159). Use levels can fall in the range of 0.5 to 20 parts per hundred on PVC in the formulation depending on the processing aid MW, the desired density, and sheet thickness. Lower density and higher sheet thickness require higher processing aid use levels.
An alternative to the use of high MW processing aids to allow foaming is to use a cross linking agent for the matrix polymer. The cross linking agent must cure at a temperature and rate similar to the decomposition of the chemical blowing agents to set the foam. This approach is used in industry to make polyurethane, epoxy foams, and the like Handbook of Polymeric Foams and Foam Technology, 2nd Ed., Hanser Publishers, Munich, (2004).
This curing approach has also been used for halogenated polymers like PVC. In a typical approach, PVC, blowing agent, and cross linking agent are combined together and placed in a mold under pressure. The mold is heated to the temperature that causes the blowing agent to generate gas and the pressure is released causing foaming and curing to occur in the same time frame. In this way, the foam structure is locked in and a thermoset material is generated that has high heat resistance and resistance to compression set, but scrap from the foam cannot easily be reprocessed. Also, this type of approach does not lend itself to extrusion type foaming processes as curing tends to occur inside the extruder.
Examples for this type of approach include U.S. Pat. No. 3,261,785 where a non-polymeric poly functional sulfonazide is used as a cross linker for PVC. In U.S. Pat. No. 4,956,222, an isocyanate curing agent is used with plasticized PVC where the PVC contains active hydrogen functionality, or an acrylic polymer with active hydrogen functionality is blended with the PVC and cured with an isocyanate. In European Polymer Journal, vol. 36, p. 2235 (2000), cross linking of PVC foam through the use of peroxides and trimethacrylate monomers is described. These approaches have the limitations that scrap cannot be reprocessed. Also, this type of approach does not lend itself to extrusion type foaming processes as controlling the curing rate so that the material does not cure in the extruder and cause melt viscosity issues is difficult.